Existing public phone systems rely primarily on land lines and microwave repeaters to handle call traffic. Recent expansions of cellular networks have enhanced phone service by providing access to the land based system for customers using mobile phones in their vehicles or hand-held portable units. The increased capacity offered to the subscriber is relatively small compared to the number of subscribers using conventional fixed phones, since cellular service is limited to only those geographical regions that are densely populated. Cellular communication is often afflicted by poor performance when customers travel from one cell to another, or when they traverse the radio shadows projected by terrain or buildings.
One previous attempt to bypass the limits of conventional communications networks utilizes a transportable telephone that employs a satellite dish several feet in diameter to communicate directly with satellites in 22,300 mile geostationary orbits. The transponders on board these satellites then connect the caller directly to the land-based system, which directs their call through switches on the ground. These devices are heavy, difficult to transport, and are prohibitively expensive.
Public phone companies do not currently offer continuous world-wide service to subscribers using a mobile or portable phone without the use of costly and large antenna systems. Commercial spacecraft and transponders that are presently on orbit do not generally possess the power capacity to communicate directly with terminals that are not coupled to an antenna dish that is at least a few feet in diameter. The service which is available is extremely limited and too expensive for use by all but a few.
An alternative to the large ground-based satellite dish is a large antenna array mounted on surfaces which are deployed from the spacecraft or satellite. Appropriate frequencies and sufficient gain in a low Earth orbit space-borne antenna would eliminate the need for a large, high-gain dish on the ground, thereby opening the way for a much lower gain antenna that can be carried by a mobile unit or by a person on a hand-held unit. Large, stowable surfaces for supporting a very high gain phased array of antenna elements, which can be accurately positioned on deployment have, until now, been unavailable.
In recent years there have been several developments in the field of deployable antenna array systems for achieving communications with Earth-based stations.
In U.S. Pat. No. 5,017,925, Bertiger et al. disclose a multiple beam space antenna system for facilitating Earth communications between a satellite switch and a plurality of Earth-based stations. The antenna is deployed after the satellite is in orbit by inflation of a raft-type supporting structure which contains a number of antenna horns. The antenna horns are mounted in concentric circular groups about a centrally located antenna horn. Each of the antenna beams projects an area on the Earth. Each of the areas of the beams are contiguous. As a result, one large area is subdivided into many smaller areas to facilitate communications. A lens may be employed to focus the antenna beams emanating from the horn antennas.
Spring et al., U.S. Pat. No. 4,901,085, discloses a communications satellite which uses a low level beam forming network connected to a hybrid matrix power amplifier section which has output ports connected to an array of radiating elements. Several beams emanate from the array and at least some of the elements are used in common for more than one beam to achieve beam overlap. Such a multibeam antenna provides high gain and frequency reuse capability.
In Scott et al., U.S. Pat. No. 4,335,388, a multibeam antenna is disclosed which produces a null at one or more specified points with greater frequency band width for a given null depth. An antenna assembly in accordance with an embodiment of this disclosure has a phase rotation means coupled to the antenna elements to adjust the phase of a signal applied to each of the antenna elements. Phase rotation nulling is used such that a narrow beam at a null point can be turned off. Thus, radiation pattern null shaping provides a null in a specified composite antenna pattern without materially changing the composite pattern in other directions. Scott et al. aver that generating nulls is advantageous to minimize the deleterious effects of discrete sources of interfering radiation which impinge on the antenna aperture while the antenna provides radio communications in other directions. In some satellite communications it may be desirable to aim a null at a jamming source while maintaining coverage of the antenna beam pattern in other angular directions.
Chen, U.S. Pat. No. 5,162,803, discloses a combination of doubly folded parallel plate beam combiners or dividers, configured to produce a desired composite beam form used in arrays of antenna elements. The doubly folded combiner or divider functions to expand a transmitted beam, or contract a received beam, in one selected plane. Use is made of a beam forming structure of this type in conjunction with an army of transmit/receive microwave modules providing amplification and phase shifting functions, and an array of printed circuit antenna elements. With appropriate phase shifting controls, a composite beam transmitted or received by the array of antenna elements can be steered independently in azimuth and elevation, using much less complex control circuitry than a conventional phased array antenna system.
Kaminskas, U.S. Pat. No. 4,811,034, discloses a plurality of small segments stowed in a compact stack and assembled, essentially automatically, into a large construction surface such as a flat or parabolic reflector. The construction surface is of a kind having surface segments of essentially identical configuration stacked along a common axis for storage in minimum space and which is assembled side by side to extend over a wide area. Each segment is hinged at a corner and pivots about the hinge to unfold. The pivot-type hinge translates in order to permit one segment to become coplanar with the others at least along one common edge. To achieve a parabolic shape for the deployed segments, the hinge is canted at an angle with respect to the axis of the undeployed stack. In order to create the curved construction surface disclosed by Kaminskas, the individual segments themselves have a curved surface and as a result require a larger volume for stacking than would be required by flat segments. The segments themselves need to be of a thickness imparting rigidity. This adds additional requirements for stowage space. It does not appear that a structure such as Kaminskas' would accommodate the antenna feed lines required for an array of ultra-high frequency (UHF) or extra-high frequency (EHF) antennas that are contemplated for direct communication with mobile or hand-held communicators.
The problem of providing an economically viable network for voice, data, and video which can be used by subscribers all over the world has presented a major challenge to the communications business. The development of a high power satellite system which can transmit and receive radio signals to portable, mobile, and fixed terminals on the land and sea and in the air without the intermediate steps of routing traffic through land-based equipment would constitute a major technological advance and would satisfy a long felt need within the electronics and telephone industries.